1. Field of the Invention
The present invention relates to a printing method for a thermal-transfer recording system using two or more kinds of thermal-transfer ink.
2. Description of Related Art
In a printing device using a thermal-transfer recording system, a clear printed matter can be obtained by selectively transferring ink, which is fluidized or sublimated by heating, onto an ordinary paper. When a plurality of thermal-transfer media is used with this kind of printing device, a color recording or gradation recording is obtainable by layering ink on the same printing paper. That is, if thermal-transfer media are made with different color inks, color recording can be obtained by using these different colors as disclosed, for example, in U. S. Pat. No. 4,388,628. On the other hand, if a plurality of inks of the same color but of different densities are prepared, gradation expression can be obtained.
FIGS. 11 and 12 are diagrams depicting the printing method and the printing device described above. A printing device shown in FIG. 11 uses roll paper on which three kinds of thermal-transfer media are formed. That is, roll paper 1 is formed by successively connecting thermal-transfer media sections of a cyanic color (C), a magenta color (M), and a yellow color (Y). Different kinds of ink are applied individually in sections of predetermined width on a single base layer (substrate). Printing paper 2 is urged against thermal head 4 by pressing roller 3 thereby bringing the printing paper 2 into contact with the roll paper 1. Under the condition described above, the thermal head 4 is selectively driven for every line, so that fluidized or sublimated ink is transferred onto the printing paper 2. During the printing operation, when roll paper 1 is subscanned in the direction of the arrow in FIG. 11, the printing paper 2 is advanced in the same direction. Upon completion of printing with one color, the printing paper 2 is rewound to the initial printing start position and a second color is printed as described above. By repeating this step, three colors are printed on the recording paper 2.
On the other hand, FIG. 12 shows a printing device in which a printing paper 7 is set on a platen 6 and thermal-transfer recording media 8-1 through 8-4 are passed successively through a portion between a thermal head 4 and the rotating platen 6. In this case, one color ink or one ink printing density is applied to the base layer by each of the thermal-transfer media 8-1 to 8-4.
With these conventional printing methods, thermal-transfer recording media used for each printing device have the same thermal characteristics. Conventionally, a thermal-transfer recording media must have a thermal characteristic such that an equal quantity of thermal-transfer ink is adhered or transferred onto a not-yet printed white paper when a thermal pulse of predetermined energy is generated from a thermal head.
In the printing method using a thermal-transfer recording system where a plurality of thermal-transfer media are used, frequently a plurality of kinds of ink are layered on a printing paper. Thermal-transfer ink is solidified such that it is adhered to, or partly permeated into, the printing paper used as a record maintaining body or it is adhered to the ink already transferred onto the paper. However, the state of adhesion, the degree of permeation, and even the quantity of transfer may vary depending on the state of the surface of the record maintaining body. Therefore, the state of transferred ink conventionally has varied between portions where the surface of the printing paper was previously uncovered and where it was covered such that color shades or darkness were irregular, or picture quality deteriorated as by disturbance in resolution or the like.